Thin film transistor having a three-portion gate electrode and liquid crystal display using the same

ABSTRACT

The present invention relates to a thin film transistor and a liquid crystal display. A gate electrode is formed to include at least one portion extending in a direction perpendicular to a gain growing direction in order to make electrical charge mobility of TFTs uniform without increasing the size of the driving circuit. A thin film transistor according to the present invention includes a semiconductor pattern a thin film of poly-crystalline silicon containing grown grains on the insulating substrate. The semiconductor pattern includes a channel region and source and drain regions opposite with respect to the channel region. A gate insulating layer covers the semiconductor pattern. On the gate insulating layer, a gate electrode including at least one portion extending in a direction crossing the growing direction of the grains and overlapping the channel region is formed. In a liquid crystal display according to the present invention, a plurality of thin film transistors forming a data driver circuit include thin films of polycrystalline silicon formed by sequential lateral solidification, at least one portion of a gate electrode of each thin film transistor extends in a direction crossing the grain growing direction, and at least one of the plurality of thin film transistors has a gate electrode having a pattern different from other thin film transistors.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation application of U.S. application Ser. No.10/500,514, filed on Dec. 3, 2004 now U.S. Pat. No. 7,183,574, thedisclosure of which is incorporated by reference herein in its entirety,and which, in turn, claims foreign priority under 35 U.S.C. § 119 toKorean Patent Application No. 2002-0000179, filed on Jan. 3, 2002, whichis hereby incorporated by reference for all purposes as if fully setforth herein.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a thin film transistor and a liquidcrystal display.

(b) Description of Related Art

A liquid crystal display (“LCD”) includes upper and lower panelsprovided with electrodes thereon and liquid crystal material interposedtherebetween. The LCD displays an image by applying electric field toliquid crystal material interposed between the two panels using theelectrodes and controlling the intensity of the electric field to adjustthe transmittance of light passing through the panels.

The most popular one among those LCDs is the one that a common electrodeand a plurality of pixel electrodes are formed on the respective panels,and a plurality of thin film transistors (“TFTs”) switching the voltagesapplied to the pixel electrodes are formed on the panel with the pixelelectrode.

The most conventional TFT used in an LCD is an amorphous silicon TFTusing amorphous silicon as semiconductor patterns.

The amorphous silicon TFT has electrical charge mobility of about0.5-1.0 cm2/V.sec, and, therefore, it can be used as a switching elementof an LCD. However, it is not proper to use the amorphous silicon TFTsfor a driving circuit directly on the liquid crystal panel due to itsinsufficient electrical charge mobility.

To overcome this problem, a polycrystalline silicon TFT usingpolycrystalline silicon having electrical charge mobility of about20-150 cm2/V.sec as semiconductor pattern is developed. Since thepolycrystalline silicon TFT has relatively high electrical chargemobility as described above, Chip In Glass structure in which drivingcircuits are embedded in the liquid crystal panel can be implemented.

Techniques for obtaining polycrystalline silicon thin film includeas-deposition technique depositing polycrystalline silicon directly on asubstrate at high temperature, a solid phase crystallization techniquedepositing amorphous silicon and crystallizing at high temperature, atechnique depositing amorphous silicon and crystallizing by laser, andso forth. However, since those techniques require a high temperatureprocess, it is not proper for application of glass substrates for LCDs.Also, they have a disadvantage that electrical characteristics are notuniform between TFTs due to non-uniform grain boundaries.

To resolve these problems, a sequential lateral solidificationtechnique, which can artificially control distribution of a grainboundary, is suggested. This technique uses the fact that the grain ofpolycrystalline silicon grows in a direction perpendicular to theboundary plane between a liquid phase region exposed to laser beam and asolid phase region which was not exposed to laser beam.

In the sequential lateral solidification technique, a laser beam passesthrough a transmission area of a mask having a slit pattern tocompletely melt amorphous silicon to form liquid phase regions arrangedin a shape of the slit pattern in the amorphous silicon layer. Then, theliquid phase amorphous silicon becomes cooled to be crystallized. Atthis time, a grain grows from the boundary of a solid phase region whichwas not exposed to laser in a direction perpendicular to the boundaryplane, and the grains stop growing when they meet at the center of theliquid phase region. Such sequential lateral solidification cancrystallize the whole thin film by moving the slit pattern of a maskalong the growing direction of the grains.

However, if the sequential lateral solidification process is performedby moving the slit pattern of the mask only along the above grantgrowing direction, the grains grow to several microns in the above graingrowing direction, but they grow just some thousands of .ANG. in adirection perpendicular to the above grain growing direction.

If the size of grain has anisotropy, electrical characteristics of TFTsformed on a substrate also have anisotropy depending on the channeldirections. That is, electrical charge mobility has a large variationbetween directions parallel and perpendicular to the above grain growingdirection, and this causes a design difficulty that all TFTs should bearranged in the same direction when TFTs are formed on the liquidcrystal panel.

Generally, a data driver circuit and a gate driver circuit incorporatedin a liquid crystal panel are arranged to be perpendicular to eachother, and, even for data driver circuits, TFTs arranged in bothtransverse and longitudinal directions are required as the circuitbecomes complicated. In this case, the above-described sequentiallateral solidification may have a big disadvantage.

Therefore, if an amorphous silicon thin film is crystallized by thesequential lateral solidification technique such that anisotropy ofcrystallization characteristic is caused, driving circuit design becomesdifficult and the size of the driving circuit becomes large due to thecomplicated wiring.

SUMMARY OF THE INVENTION

The present invention is to make electrical charge mobility of TFTsuniform without increasing the size of a driving circuit in an LCD.

To achieve the above object, a gate electrode is formed such that atleast one portion of the gate electrode extends in a directionperpendicular to a grain growing direction.

Specifically, a thin film transistor according to the present inventionincludes a semiconductor pattern including a thin film ofpoly-crystalline silicon containing grown grains on an insulatingsubstrate. The semiconductor pattern includes a channel region andsource and drain regions opposite with respect to the channel region. Agate insulating layer covers the semiconductor pattern. On the gateinsulating layer, a gate electrode including at least one portionextending in a direction crossing the growing direction of the grainsare formed to overlap the channel region.

Here, the at least one portion of the gate electrode may cross thegrowing direction of the grain at right angle. The gate electrode mayinclude a first portion extending in a direction parallel to the growingdirection of the grains and second and third portions connected torespective ends of the first portion and extending in a directionperpendicular to the growing direction of the grains. Alternatively, thegate electrode includes a first portion extending in a directionperpendicular to the growing direction of the grains and second andthird portions connected to respective ends of the first portion andextending in a direction parallel to the growing direction of thegrains.

A liquid crystal display according to the present invention includes adisplay area defined on the insulating substrate for displaying picture,a data driver circuit for transmitting data signals to the display area,and a gate driver circuit for transmitting gate signals to the displayarea. Here, the data driver circuit includes a plurality of thin filmtransistors including a thin film of polycrystalline silicon formed bysequential lateral solidification. Each thin film transistor includes agate electrode including at least one portion extending in a directioncrossing the growing direction of grains. The gate electrode of at leastone of the plurality of thin film transistors has a pattern differentfrom other thin film transistors. It is preferable that the gate drivercircuit includes a plurality of thin film transistors including a thinfilm of polycrystalline silicon formed by sequential lateralsolidification, each thin film transistor includes a gate electrodeincluding at least one portion extending in a direction crossing thegrowing direction of grains, and the gate electrode of at least one ofthe plurality of thin film transistors has a pattern different fromother thin film transistors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a sequential lateralsolidification process;

FIG. 2 schematically shows a detailed structure of a grain in apolycrystalline silicon thin film formed by a sequential lateralsolidification;

FIGS. 3A and 3B show schematic structures of TFTs according to first andsecond embodiments of the present invention; and

FIG. 4 is a schematic diagram of an LCD according to an embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the present invention is described in detail with reference toaccompanying drawings.

First, a technique of crystallization of an amorphous silicon thin filmusing sequential lateral solidification is described.

FIG. 1 is a schematic diagram showing a sequential lateralsolidification process, and FIG. 2 schematically shows a detailedstructure of a polycrystalline silicon thin film during crystallizationfrom amorphous silicon to polycrystalline silicon in the sequentiallateral solidification process.

As shown in FIG. 1, according to the sequential lateral solidificationprocess, a laser beam is applied to a plurality of local regions of anamorphous silicon layer 200 formed on an insulating substrate using amask 300 having a transmission area 310 with a slit pattern tocompletely melt the amorphous silicon in the local regions such that aplurality of liquid phase regions are formed in an area of the amorphoussilicon layer 200 corresponding to the transmission area 310.

At this time, a grain of polycrystalline silicon grows from a boundarysurface between the liquid phase region 210 exposed to the laser beamand a solid phase region 220 where the laser beam is not applied along adirection perpendicular to the boundary surface. The grains stop growingwhen they meet at the center of the liquid phase region. They are grownto a desired degree by applying a laser beam while moving the slitpattern of the mask 300 along the growing direction of the grains tocontinue the lateral growth of the grains. Therefore, the size of thegrain can be controlled.

FIG. 2 shows a grain structure of polycrystalline silicon formed by thesequential lateral solidification process using a mask provided with aslit pattern elongated in a transverse direction.

It can be known that grains are grown in a direction perpendicular tothe slit pattern. It can be also known that the polycrystalline siliconthin film shown in FIG. 2 is formed by the sequential lateralsolidification process using two slit patterns.

“L” is a boundary where two adjacent polycrystalline silicon regionsmeet when the, polycrystalline silicon regions are formed by performingthe sequential lateral solidification using respective slit patterns.

However, if the sequential lateral solidification process is performedwhile moving the slit pattern of the mask only along the grain growingdirection, grains of several microns are obtained in the grain growingdirection, but small grains of some thousands .ANG. are formed in adirection perpendicular to the grain growing direction.

If a gate electrode runs across a semiconductor layer of a thin filmtransistor along a direction perpendicular to the grain growingdirection, the direction of a channel formed in the semiconductor layerof the TFT is parallel to the growing direction and thus the electronmobility passing through the channel is high. On the contrary, if a gateelectrode runs parallel to the grain growing direction, the channeldirection is perpendicular to the grain growing direction and thus theelectron mobility passing through the channel is low. The difference inthe electron mobility is generated since electrons moving in the channelmove within the grains without directly passing through the grainboundary.

The electron mobility of the TFT has a large variation depending on therunning direction of the gate electrode across the semiconductor layer,and characteristics of the TFTs formed on the liquid crystal panelbecome very irregular depending on their positions. To solve theseproblems, at least a portion of the gate electrode of the TFT accordingto an embodiment of the present invention extends in a directioncrossing the grain growing direction.

Then TFTs according to embodiments of the present invention, which areprovided with polycrystalline silicon thin films formed by thesequential lateral solidification, will be described.

FIG. 3A is a layout view showing a designed arrangement of asemiconductor pattern and a gate electrode of a TFT according to a firstembodiment of the present invention.

In the TFT according to this embodiment, a semiconductor pattern 10 hasa rectangular shape with a length parallel to a grain growing direction.

A gate electrode G formed at the semiconductor pattern 10 is designedsuch that portions of the gate electrode G extend perpendicular to thegrain growing direction and thus electrons passing through a region of achannel of the semiconductor pattern 10 corresponding to this portionmove along the grain boundary without running into the grain boundary.

In this embodiment, the gate electrode G includes a first portion G1extending parallel to the grain growing direction, and second and thirdportions G2 and G3 connected to respective ends of the first portion G1and extending perpendicular to the grain growing direction. Thesemiconductor pattern 10 includes a source region S and a drain region Ddoped with conductive impurities located opposite with respect to thegate electrode G.

When a gate-on voltage is applied to the gate electrode G, a channelwhere electrons can move is formed in a portion of the semiconductorpattern 10 under the gate electrode G. When a data voltage is applied tothe source region S under this condition, electrical charge in thesource region S moves to the drain region D through the channel. At thistime, the electrical charges pass straight through portions of thechannel formed by the second and the third portions G2 and G3 of thegate electrode G along paths having no grain boundary, and thus themobility becomes high. Arrows in the figures indicate the movement ofelectrical charge.

FIG. 3B is a layout view schematically showing the structures of asemiconductor pattern and a gate electrode of a TFT according to asecond embodiment of the present invention.

In the TFT according to this embodiment, a semiconductor pattern 10 hasa rectangular shape with a length perpendicular to a growing directionof grains.

A gate electrode G formed at the semiconductor pattern 10 is designedsuch that a portion of the gate electrode G extends perpendicular to thegrain growing direction and thus electrons passing through a region of achannel of the semiconductor pattern 10 corresponding to this portionmove without running into the grain boundary.

In this embodiment, the gate electrode G includes a first portion G1extending perpendicular to the grain growing direction, and second andthird portions G2 and G3 connected to respective ends of the firstportion G1 and extending parallel to the grain growing direction. Thesemiconductor pattern 10 includes a source region S and a drain region Ddoped with conductive impurities located opposite with respect to thegate electrode G.

When a gate-on voltage is applied to the gate electrode G, a channelwhere electrons can move is formed in a portion of the semiconductorpattern 10 under the gate electrode G. When a data voltage is applied tothe source region S under this condition, electrical charge in thesource region S moves to the drain region D through the channel. At thistime, the electrical charges pass straight through portions of thechannel overlapping the first portion G1 of the gate electrode G alongpaths having no grain boundary, and thus the mobility becomes high.Arrows in the figures indicate the movement of electrical charge.

Although the above-described exemplary TFTs according to the first andsecond embodiments of the present invention includes the gate electrodesincluding a portion extending perpendicular to the grain growingdirection, the electron mobility may become higher if there is no grainboundary which obstructs the movement of the electrons in the channel.Therefore, a TFT according to the present invention is formed such thata portion of the gate electrode G extends in a direction crossing thegrain growing direction and thus a region of the channel formed by thatportion has no grain boundary.

As described above, by improving the structure of a gate electrode suchthat at least a portion of the gate electrode extends in a directioncrossing the grain growing direction, the electrons can move swiftlythrough the channel of the semiconductor pattern formed thereunder.

In general, since electrical characteristics of a TFT are determined byhighest electrical charge mobility, TFTs having gate electrodesincluding at least a portion extending in a direction crossing the graindirection according to the present invention have relatively uniformelectrical charge mobility regardless of the shape of the gateelectrodes and semiconductor patterns. In addition, TFTs having intendedelectrical characteristics can be obtained by adjusting the widths andthe lengths of the gate electrodes, which determines the electricalcharacteristics of the TFTs.

Current characteristic of a P-type TFT designed according to theembodiments of the present invention was measured.

A TFT was designed such that its gate electrode extends parallel to agrain growing direction and includes at least a portion crossing thegrain growing direction at right angle according to the first and secondembodiments of the current invention. Thereafter, the current mobilityof the TFT was measured, and measured current mobility is improved byequal to or more than 30% compared with a TFT having a gate electrodeentirely extending parallel to the grain growing direction.

FIG. 4 is a schematic diagram of an LCD according to an embodiment ofthe present invention, which shows an arrangement of TFTs in a gatedriver circuit and a data driver circuit.

The LCD includes a display area 101 having a plurality of pixelsarranged in a matrix on an insulating substrate 100 for displaying imageand a data driver circuit 102 and a gate driver circuit 103 for applyingdata signals and gate signals to the display area 101, respectively.Here, semiconductor patterns I, II and III of TFTs formed in the displayarea 101, the data driver circuit 102, and the gate driver circuit 103include polycrystalline silicon thin films formed on the insulatingsubstrate 100 by the sequential lateral solidification.

The semiconductor patterns I, II and III of each driving circuit 102 or103 have different shapes depending on available areas or wiring design.

As shown in the figure, the semiconductor patterns I and II of the firstand the second TFTs of the data driver circuit 102 are rectangular andextend perpendicular or parallel to the grain growing direction as inthe first and the second embodiments of the present invention. Thesemiconductor pattern III of a third TFT is formed oblique to the graingrowing direction, but its gate electrode G is patterned such that atleast one portion part of the gate electrode, i.e., a portion denoted as“A” crosses the grain growing direction, for example, at right angle toenhance the electrical charge mobility. The semiconductor patterns I,II, and III of the first, second and third TFTs in the data drivercircuit 102 are mere example, and they can be formed to have variouspatterns and shapes. Here, at least one of a plurality of TFTs formingthe data driver circuit 102 may have a gate electrode having a patterndifferent from the other TFTs.

Furthermore, a plurality of TFTs (not shown) in the gate driver circuit103 may also have various shapes as described for the data drivercircuit 102.

Since the TFTs in the display area 101 do not need high electricalcharge mobility, silicon thin films used as semiconductor patterns canbe selected from amorphous silicon, polycrystalline silicon formed by aconventional crystallization technique such as high temperaturecrystallization or laser crystallization, and polycrystalline siliconformed by sequential lateral solidification. At this time, it ispreferable that a plurality of TFTs in the display area 101 have thesame condition to ensure uniform electrical characteristic in thedisplay area 101.

TFTs according to an embodiment of the present invention is manufacturedby a conventional TFT manufacturing method, and a polycrystallinesilicon thin film to form a semiconductor pattern may be formed usingthe sequential lateral solidification in the same way.

In the present invention, the shape of the gate electrode is notconfined to the suggested embodiment, and the gate electrode can ratherhave various shapes regardless of the shape of the semiconductor patternas long as a portion of the gate electrode extends in a directioncrossing the grain growing direction.

TFTs used to form elements of the gate driver circuit or data drivercircuit of an LCD may include semiconductor patterns having propershapes depending on the position. Also, the directions of the channelscan be controlled in this case, thereby removing wiring complexity.

According to the present invention, a TFT having high electrical chargemobility and uniformity of electrical charge mobility between TFTs canbe obtained without enlarging the size of the driving circuit

1. A thin film transistor comprising: a crystalline semiconductorpattern formed on an insulating substrate and comprising a channelregion and source and drain regions opposite with respect to the channelregion, and a gate electrode overlapping with the channel region andcomprising a first portion, a second portion on the channel regionconnected to one end of the first portion and a third portion on thechannel region connected to another end of the first portion, the secondportion and the third portion extending in a different direction from adirection of the first portion.
 2. The thin film transistor of claim 1,wherein the crystalline semiconductor pattern has a plurality of growngains, and the first portion extends in a direction crossing a growingdirection of the grains.
 3. The thin film transistor of claim 1, whereinthe crystalline semiconductor pattern has a plurality of grown gains,and the second portion and the third portion extend in a directioncrossing a growing direction of the gains.
 4. A liquid crystal displaycomprising: an insulating substrate, a display area comprising aplurality of pixels arranged on the insulating substrate for displayingan image, a data driver circuit defined on the insulating substrate fortransmitting data signals to the display area, and a gate driver circuitdefined on the insulating substrate for transmitting gate signals to thedisplay area, wherein at least one of the data driver circuit and thegate driver circuit comprises a plurality of thin film transistors, eachof the plurality of thin film transistors comprises a crystallinesemiconductor pattern comprising a channel region and source and drainregions opposite with respect to the channel region, and a gateelectrode overlapping with the channel region and comprising a firstportion, a second portion on the channel region connected to one end ofthe first portion and a third portion on the channel region connected toanother end of the first portion, the second portion and the thirdportion extending in a different direction from a direction of the firstportion, and the gate electrode of at least one of the plurality of thinfilm transistors has a pattern different from gate electrodes of otherthin film transistors.
 5. The liquid crystal display of claim 4, whereinthe crystalline semiconductor pattern has a plurality of grown grains,and the first portion extends in a direction crossing a growingdirection of the gains.
 6. The liquid crystal display of claim 4,wherein the crystalline semiconductor pattern has a plurality of growngains, and the second portion and the third portion extend in adirection crossing a growing direction of the gains.